Preheat the oven to 350 F and line an 8x8-inch baking pan with baking paper.
Whisk together the dark cocoa powder, the plain flour, and the sea salt. Set to one side.
In a large bowl, whisk together the melted butter with the granulated sugar and the light brown sugar until it is thoroughly combined, and looks smooth and glossy.
Whisk in the 2 eggs and the vanilla extract.
Fold the dry mixture into the wet mixture using a wooden spoon until it is just combined.
Add the diced rhubarb, the dark chocolate chips, and the chopped hazelnuts, and gently fold them in, being careful to not over-mix the batter.
Transfer the batter to the prepared baking tray, smooth it out, and then place into the oven to bake for 20–25 minutes.
Remove from the oven when a skewer inserted into the brownie comes out coated with a little moist batter, and then allow the brownie to sit and cool completely before cutting and serving.